The conventional practice of assembling such sound generators is to clamp the membrane between the housing rim and a generally disk-shaped structure forming part of the resonator, that structure usually consisting of nonmetallic material such as a thermoplastic resin which can be easily molded into the requisite shape. The resonator structure defines a plenum chamber which is open toward the membrane and communicates with the air channel so that standing waves generated in that chamber by the oscillating membrane give rise to a vibrating air column in the channel. The latter often is of spiral shape, defined by a convoluted shell of the resonator structure.
In prior-art sound generators of this type the membrane has a marginal zone clamped between confronting flanges of the coil housing and the resonator structure which are bolted or riveted to each other. In order to insure a firm grip on the membrane periphery while still allowing its central portion to vibrate with the necessary stroke amplitude, the flanges must be substantially nondeformable under the stresses locally generated by the peripherally spaced fasteners. Thus, the housing flange is generally given a thickness exceeding that required for the completion of the magnetic circuit while the resonator structure is reinforced by an annular metallic disk bounding the plenum chamber. The production of such a disk, usually by deep-drawing from a sheet, is wasteful since a large central part must be removed in order to give access to the air channel; also, the assembly of the several components referred to is laborious and time-consuming.
In my above-identified prior U.S. Pat. No. 4,134,200 I have disclosed an improved sound generator of this type in which the peripheral housing wall has a rim bent back upon itself to form an outwardly projecting two-ply flange enfolding a marginal zone of the membrane, with radial separation of a bight portion of the flange from the membrane periphery to give that membrane a certain mobility.